1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method thereof, and a semiconductor device and a display device which include the thin film transistor.
2. Description of the Related Art
Thin film transistors (hereinafter also referred to as “TFTs”) are already widely used in a field of liquid crystal displays. A TFT is a kind of field-effect transistor and is named after the fact that a thin semiconductor film for forming a channel is formed. At present, a technique for manufacturing a TFT using amorphous silicon or polycrystalline silicon as the thin semiconductor film has already been put into practical use.
A semiconductor material called “microcrystalline silicon” has been known for a long time together with amorphous silicon and polycrystalline silicon, and there has also been a report on microcrystalline silicon related to a field-effect transistor (e.g., see Patent Document 1: U.S. Pat. No. 5,591,987). However, TFTs using microcrystalline silicon has been buried between an amorphous silicon transistor and a polycrystalline silicon transistor up to today; thus, there has been a delay in practical use, and reports thereof are made merely at an academic society level (e.g., see Non-Patent Document 1: Toshiaki Arai et al., “SID '07 DIGEST” 2007, pp. 1370-1373).
A microcrystalline silicon film can be formed over a substrate made of glass or the like, by decomposing source gases with plasma (weakly-ionized plasma) by a plasma CVD method; however, it has been considered that it is difficult to control generation of crystal nuclei and crystal growth because reaction proceeds in a non-equilibrium state.
Various researches have been made on microcrystalline silicon. According to a hypothesis, growth mechanism of microcrystalline silicon is as follows: first, a portion of an amorphous phase, in which atoms are aligned randomly, grows over a substrate, and then nuclei of crystals start to grow (see Non-Patent Document 2: Hiroyuki Fujiwara et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 41, 2002, pp. 2821-2828). In Non-Patent Document 2, it is considered that the density of microcrystalline silicon nuclei can be controlled with the concentration of a hydrogen gas used in forming a film because peculiar silicon-hydrogen bonds are observed on an amorphous surface when nuclei of microcrystalline silicon start to grow.
Further, influence on a growing surface of a microcrystalline silicon film due to an impurity element such as oxygen or nitrogen has also been considered. There is knowledge that by reducing the concentration of the impurity element, the crystal grain diameter of a microcrystalline silicon film is increased, and thus the defect density (especially, the defective charge density) is reduced (see Non-Patent Document 3: Toshihiro Kamei et al., “Japanese Journal of Applied Physics (Jpn. J. Appl. Phys.)” vol. 37, 1998, pp. L265-L268).
Further, there is a point of view that in order to improve operation characteristics of a TFT, the purity of a microcrystalline silicon film should be improved, and a microcrystalline silicon film in which the concentrations of oxygen, nitrogen, and carbon are 5×1016 cm−3, 2×1018 cm−3, and 1×1018 cm−3, respectively, and effective mobility is improved was reported (see Non-Patent Document 4: C.-H. Lee, et al., “International Electron Devices Meeting Technical Digest (Int. Electron Devices Meeting Tech. Digest), 2006, pp 295-298). In addition, a microcrystalline semiconductor film in which a deposition temperature by a plasma CVD method is 150° C., the concentration of oxygen is reduced to 1×1016 cm−3, and effective mobility is improved was reported (see Non-Patent Document 5: Czang-Ho Lee et al., “Applied Physics Letters (Appl. Phys. Lett.), vol. 89, 2006, p 252101).